endogenousopioidsystemisinvolvedinacuteresponsetonicotineandthedevelopmentand maintenanceofnicotinedependence.Accordingly,onedrugbeinginvestigatedforsmokingcessation treatmentisnaltrexone,aopioidreceptorantagonist.However,recentclinicaltrialsexaminingtheefficacyof adjunctivetreatmentwithnaltrexoneinsmokingcessationhaveyieldedmixedresultsand,therefore,further researchisneeded.Ofpotentialrelevancetotheseclinicaloutcomes,boththeendogenousopioidsystemand nicotinehaveactiononthehypothalamicpituitaryadrenal(HPA)axis,asystemthatisprimarilyassociated withresponsetostress.SmokershaveanacutelyunderresponsiveHPAaxistostressorsor pharmacologicalactivationcomparedtononsmokers.Importantly,thisHPAaxisdysfunctionmayplayarole inmaintenanceofaddictionandsusceptibilitytorelapse.NaltrexoneacutelyactivatestheHPAaxisandithas beenpreviouslyhypothesizedthatthisincreaseinHPAaxisactivitymayunderlienaltrexone'sefficacyfor treatmentofalcoholismandpossiblysmoking.However,therelationshipbetweennaltrexone'sactiononthe HPAaxisanditsreductionofsmokingbehaviorhasnotbeeninvestigated.Finally,theremaybeimportant,yet unexploredsexdifferencesinsmokers'responsetonaltrexone.Womensmokershaveshownmoreacute sensitivitytonaltrexonethanmenintermsofsubjective,behavioral,andHPAaxishormonemeasures,aswell asbetterquitratesinsmokingcessationtrialsusingnaltrexone.Thesereportedsexdifferencesmaybe relatedtodifferencesinlevelsofestradiol,thepredominantestrogeninhumans,betweenmenandwomen.In women,fluctuationsinestradiollevelsacrossthemenstrualcyclemayaffecttheamountofavailableopioid receptorsand,therefore,mayaffecttheefficacyofnaltrexone.Thestudiesinthisapplicationaredesignedto testtheeffectsofnaltrexoneontheHPAaxisandsmokingbehaviorinmaleandfemalesmokers,andto examinehowestradiollevelaffectstheseresponsestonaltrexoneinfemalesmokers.Inordertoexaminethe effectofestradiolonresponsetonaltrexone,womenwillbetestedattwodifferentstagesofthemenstrual cycle:theearlyfollicularphase(thelowestlevelofestradioloccurringduringthemenstrualcycle)andthelate follicularphase(thehighestoccurringlevelofestradiol).Theresultsofthisstudywouldpotentiallyprovide evidenceforestradiollevelasamechanismunderlyingsexdifferencesinresponsetonaltrexoneandmay havetranslationalclinicalimplicationsfortheuseofnaltrexoneinsmokingcessation. PUBLIC HEALTH RELEVANCE: The proposed study relates to public health in that it will investigate how men and women smokers may differ in response to a drug (naltrexone) that is being examined for use in smoking cessation. The results of this study may have direct implications in the manner that women (or men) are treated using naltrexone.